Hans Berger’s “brain mirror”

Hans Berger’s invention of the electroencephalogram (EEG) in 1924 has been described as “one of the most surprising, remarkable, and momentous developments in the history of clinical neurology”. Yet at the time the German medical and scientific establishments met his findings with incredulity and derision. Unsure of his findings himself, he did not publish his first paper on the work until five years after his discovery.

Berger’s interest in brain activity was triggered by a bizarre event during a year’s service in the German cavalry. He was part way through a mathematics degree at the University of Jena, with a view to becoming an astronomer, at the time. Thrown from a horse into the path of a horse-drawn cannon, Berger contemplated certain death, but the horses were halted in time. His sister, at home many miles away, had a feeling that Berger was in danger and insisted their father telegram him. Berger was convinced that this event was evidence of telepathy and on completing his military service he returned to Jena to study medicine with the aim of discovering the physiological basis of “psychic energy”.

On graduation Berger joined the university’s psychiatric clinic, where he stayed until retirement in 1938. Berger first recorded changes in cerebral blood flow in patients with cranial defects using a plethysmograph (an instrument that measures changes in volume within an organ). Later, he moved on to measure the energy converted into heat and electricity during various mental tasks. It was this recording of the brain’s electrical current from the surface of the scalp that led Berger to his hirnspiegel (brain mirror) — the human EEG.

Berger’s electroencephalograph used pairs of electrodes placed on the scalp to transmit a signal to one of its several recording channels. The signal conveyed the difference in voltage between the pair, and the rhythmic fluctuation of the difference was shown as waves on a line graph.

Despite scepticism from colleagues, Berger pioneered the theory and applications of EEG in many neurological and psychiatric diseases. The scientific establishment only fully accepted the human EEG after the work was replicated by the Cambridge physiologist Lord Adrian in 1934. By 1938, when Berger was made professor emeritus in psychology, EEG had gained widespread recognition and had become a practical diagnostic tool.

After a long period of depression, Berger committed suicide 70 years ago on 1 June 1941.